This invention relates to a method of hot tapping type branching, and more particularly to a method of connecting a branch pipeline to a main pipeline which is transmitting fluid such as town gas and establishing communication therebetween by hot tapping the main pipeline, i.e. perforating the main pipeline while in service, and to a split joint to be used in this method, wherein the two pipelines have an identical or like diameter.
In the most basic of known methods of this type, a bypass pipe is used to intercommunicate two points of the main pipeline and shut off and remove a main pipeline portion between the two points, then a T-shaped pipe is mounted between the two points and a branch pipeline is connected to the T-shaped pipe. Such a prior art method had many problems in that it required large scale bypassing work and troublesome pipe shutting work which also entailed excavation over large areas and were time-consuming.
The following two methods were developed in recent years to alleviate the above problems:
(I) A method used where the pressure is low: As shown in FIG. 10 of the accompanying drawings, a joint A' having a slide shutter 30 is mounted on a main pipeline B which is perforated by a hole saw 31 of a drill D' attached to the joint A'. Thereafter the shutter 30 is closed and the drill D' is replaced by a coupling pipe E' which connects a branch pipeline E to the joint A' as shown in FIG. 11. This method is executed in the following order:
(i) A gastightness test is carried out on a newly installed section Y comprising a pre-connected series of branch pipes E.
(ii) After a gastightness test for an outlet section X at the main pipeline B and the joint A', the drill D' is attached to perforate the main pipeline B.
(iii) The outlet section X and the newly installed section Y are brought into intercommunication through a coupling section Z comprising the coupling pipe E'.
(II) A method used where the pressure is high: As shown in FIG. 12, this method employs a joint A" having no slide shutter and a shutting device having strong resistance to pressure such as a valve C', and is executed in the same manner and the order as the method (I).
These two methods(I) and (II) dispensed with the work of laying a bypass for the main pipeline B, the main pipeline shutting operation and the removal of a portion of the main pipeline B, and greatly alleviated the problems of the prior method. However, in both of these methods, as clearly seen from the split joints A' and A" used therein, the perforating work and the connection of a branch pipeline are carried out using a common opening and in the same direction, and therefore the perforating work requires a considerably large space in the branching-out direction. Particularly in the case of underground pipelines, a large area has to be excavated and, when there is another pipeline nearby, the required space can hardly be secured. This is the disadvantage of the methods (I) and (II).
The method (I) has a further disadvantage that, in addition to troublesome final connection work, the gastightness test of the coupling section Z cannot be conducted with a pressure higher than that of the transmitted fluid, making it difficult to provide the test with a high degree of reliability. The method (II) permits the newly installed section to be pre-connected, but leaves the valve C' at the outlet section X usually located under an intersection of roads, which poses a problem of maintenance.
Taking a view that these varied problems spring from the fact that the perforating work and the branching work are carried out in the same direction, a further method has recently been provided wherein the two steps above are carried out in different directions, which is described with reference to FIG. 13. As seen, a main pipeline B is fitted with a split joint OA including a drill receiving opening O4 and an outlet opening O5 for coupling to a branch pipeline E, the two openings having axes crossing each other at right angles. After coupling the branch pipeline E to the outlet opening O5, drill OD is fixed to the drill receiving opening O4 across a shutting device C, and then a peripheral wall of the main pipeline B opposed to the outlet opening O5 is cut off by a hole saw O9 of the drill OD thereby establishing communication between the main pipeline B and the branch pipeline E. Thereafter the shutting device C is removed and the drill receiving opening O4 is closed by screwing in a plug and placing a blind flange. Because the perforating work and the branching work are carried out in different directions according to this method, the perforating work no longer requires a large space in the branching-out direction as was the case with the prior methods (I) and (II). Also, because the branch pipeline E is connected prior to the perforating work, the entire branch pipeline E, including the split joint O4, can be subjected to a gastightness test using a higher pressure than the fluid pressure inside the main pipeline B. In addition to the above advantages, this method permits the shutting device C to be removed, whereby its maintenance is no longer necessary.
However, the perforating work according to this method has a problem which may more than offset the above advantages, and therefore it is of primary importance to solve this problem for its practical application.
The problem lies in the mode of the perforating work in which the hole saw O9 cuts the peripheral wall of the main pipeline opposed to the outlet opening O5 gradually from one peripheral point to another, that is to say the hole saw O9 operates in a position greatly displaced from a plane crossing the axis of the main pipeline B. Thus, at the initial stage of cutting a forward end of the hole saw O9 is prone to slipping on the surface of the main pipeline B and is subjected to a strong reaction force in a sideways direction away from the main pipeline B, which hamper the perforating work itself, result in a perforation area different from what is desired, and causes damage or other trouble to the hole saw or its rotating mechanism through overstrain.
As noted above, the prior methods (I) and (II) and the method proposed recently of hot tapping type branching and the split joints used therein have varied problems.